An organic EL (Electro Luminescence) display has drawn attention as a display of the next generation because of its self-emission, fast response, brightness and wide viewing angle. Among other things, an active matrix organic EL display is applicable for various purposes from a cell phone to a large-sized TV set because it can be manufactured in higher definition, and there are great expectations for the organic EL display.
Organic EL elements that form pixels need drive elements for controlling a current flown in the organic EL elements in order to control emission. A TFT (Thin Film Transistor), for example, is used as the drive element, and a low-temperature polysilicon TFT in particular is considered to be appropriate as the drive element for driving the organic EL elements since it has relatively high mobility, is operable at high-speed, and is stable for a relatively long time.
As described, although the low-temperature polysilicon TFT is stable and has high mobility, uneven luminance easily occurs when it is used in a saturated region because its characteristics are not uniform. Herein, uniformity can be improved when the TFT is used as a switch and a digital drive, which generates gradation depending on whether or not voltage is applied to the organic EL elements, is used.
However, in this case, since the organic EL elements are controlled depending on whether or not voltage is applied, it has a drawback that burn-in tends to appear on a display due to the degradation of organic EL elements associated with a long-time operation, that is, they become highly resistive.
Further, since the current-voltage characteristics of the organic EL elements changes when the ambient temperature changes, a larger amount of current flows when the temperature rises, for example, even if the same voltage is applied. If the amount is different for red (R), green (G) and blue (B) in the case of full-color display, white balance becomes off-balance and there is a problem that the original color cannot be expressed.